It is known that for the installation of cables, for example for telephones, railroads, high or low tension, for fiber optic communication or otherwise, arranged aerial or underground, considerable traction forces are required which may be dangerous to apply due to the accidental obstacles that can increase the normal sliding friction.
Laying operations are particularly difficult in the case of stringing cables on long-distance electric power lines.
In order to effect such operations, hydraulic winches are generally used, consisting of a thermal motor able to drive a hydraulic pump which in turn drives a respective hydraulic motor which determines the winding of the line that draws the cable.
Such known hydraulic apparatuses have a main disadvantage, however, which is that they have a hydraulic plant regulated only by a so-called “limit valve”. Such valve makes the oil re-circulate, bypassing the pump, when the working pressure in the circuit exceeds a pre-determined value which entails excessive, or in any case dangerous, traction of the cable, due to the resistance of the structure of the latter.
In known apparatuses, the working pressure of the hydraulic circuit depends on the reaction offered to the sliding of the cable itself. Therefore, in the event of a sudden and accidental obstacle to the sliding, due for example to a guide pulley seizing, the hydraulic motor slows down and therefore there is a rise in the hydraulic pressure.
Such increase in pressure is discharged onto the hydraulic motor, which thus exerts very high and dangerous traction, which can even lead to breakage of or damage to the cable being drawn.
In such operating conditions, the recirculation valve keeps the pressure at pre-determined values, thus entailing a high transformation of the mechanical power yielded by the thermal motor, in heat, to effect the recirculation of almost all the oil. Such transformation entails an overheating of the oil, however, causing it to almost totally lose its lubrication characteristics, and damaging the rubber or plastic parts, which are sensitive to heat.
These problems are even more relevant in the case of laying cables with fiber optics, which require a particular caution in use and a precise control of the axial stress load, in order to prevent them from being ruined.
One purpose of the present invention is to achieve a laying apparatus, and perfect a laying method, for cables and suchlike, which, in the event of a sudden and accidental obstacle to the sliding of the cables, does not entail an overheating of the oil due to the recirculation of the latter.
Another purpose of the present invention is to achieve an apparatus with reversible parts which can, if necessary, be used as a brake.
A further purpose is to automate the intervention to control the overheating of the oil.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.